Thanks for the comments and explanations as to the TPS workings - helps if you understand the basics of how these things work.
Inflated all tyres back up to 2.4 bar, didn't reset the button, but all seems fine from a mile out to test it. I'll do the button re-set after the MOT, - don't want to tempt fate - ha ha. I didn't realise that it worked detecting if one wheel was running different to the others, but that makes sense.
My tyres are legal, but I must change them soon - giving me chance to shop around, rather than as part of the MOT
Cheers guys